The present invention relates to a method of making a heater element. More particularly this invention concerns the manufacture of such an electrical heating element used in an injection-molding machine.
An electrical heating element for an injection-molding machine or the like has a normally helical resistive wire that is surrounded by an insulating mass and held in a metallic tubular casing. The ends of the tubular casing are fitted with dielectric caps carrying terminals connected to the ends of the resistive wire so electricity can be passed through the wire to heat it. This heat is transmitted through the insulating mass, which is a dielectric and not thermal insulation, to the tubular casing and thence to the part the heater is fitted to. Such devices are mass produced and are subject to considerable wear and tear so they must be replaced periodically. In addition these heaters often need to be shaped to fit around a particular part or in a groove of a part.
Such a heater is typically made by assembling all but the cap and terminal at one end of the heater. The insulation mass is poured into the open end of the tube to fill it, and the other cap and terminal are installed to complete the assembly. If the mass is too loose there is poor heat conduction and the wire can directly contact the surrounding metal casing. It has been suggested therefore to somewhat flatten the heater to compact this mass, but such flattening results in a shape that is difficult to shape and diametral flattening produces nonuniform compaction of the mass in the casing in any case.
If the casing is smooth it is likely to kink when bent, so German 42 42 505 suggests making the tubular casing corrugated. This, however, makes it very difficult to fill densely and makes it impossible to radially compact the casing later. In another system described in German patent document 195 41 504 the insulating mass is a dielectric but heat-conducting metal powder and the casing is smooth. The smooth inner casing in turn is surrounded by a corrugated outer sleeve and the space between them is filled with another insulating mass. Such a system is fairly effective as it allows at least the inner mass to be compacted, but manufacture is fairly expensive.
It is therefore an object of the present invention to provide an improved method of making an electrical heating element.
Another object is the provision of such an improved method of making an electrical heating element which overcomes the above-given disadvantages, that is which is fairly simple and inexpensive to carry out but which produces an electrical heating element that can be readily bent and that has a densely compacted insulating mass between the resistive wire and the surrounding casing.
A method of making a heater element has according to the invention the steps of inserting into a smooth tubular casing a heating wire, filling a space between the wire and the casing with a compressible insulating mass, and closing ends of the space to confine the mass in the casing. Then according to the invention the casing is radially inwardly compressed to radially compress the insulating mass and also to form in the casing a successive of radially outwardly open annular grooves extending circumferentially fully around the casing.
By radially compressing the casing to form the bendfacilitating grooves in it after filling it with the insulating mass, excellent and perfectly uniform compression of this mass is insured. Thus in a single simple manufacturing procedure, typically done by passing the assembled heater between a pair of appropriately shaped rollers, it is possible both to compact the mass in it and form it with the grooves necessary to make it easy to bend. The resultant heater can be made at the same cost as a prior-art heater but will be of substantially higher quality.
According to the invention the casing is formed with a succession of such grooves, which can be of V-shape and which are each endless so that the casing is corrugated. Alternately the grooves are interconnected helically. Either way in accordance with the invention the grooves form axially relatively long and radially relatively thick outer rings and axially relatively short and radially relatively thin inner rings. The inner rings deform readily so that the heater can be given virtually any desired shape.